Consolidated Statements of Cash Flows	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Cash Flows from Operating Activities
Net income	$ 271,885,000	$ 176,042,000	$ 175,099,000
Adjustments to reconcile net income to net cash provided by operating activities:
Reversal for credit losses	(4,500,000)	(2,500,000)	(15,650,000)
Provision for losses on other real estate owned		691,000	176,000
Deferred tax provision	2,343,000	34,554,000	15,949,000
Depreciation and amortization	7,314,000	7,265,000	7,490,000
Net gains on sale and transfers of other real estate owned	(1,187,000)	(2,661,000)	(546,000)
Net gains on sale of loans			(285,000)
Proceeds from sale of loans	8,000,000	7,500,000	20,079,000
Originations of loans held for sale			(12,665,000)
Loss on sales or disposal of fixed assets	107,000
Amortization of alternative energy partnerships, venture capital and other investments	40,871,000	4,572,000	28,897,000
Write-down on impaired securities			206,000
Gain on sales and calls of securities	(22,000)	(1,006,000)	(5,104,000)
Other non-cash interest income	(1,576,000)	(1,740,000)	(1,272,000)
Amortization of security premiums, net	3,018,000	2,945,000	6,371,000
Unrealized loss on equity securities	2,787,000
Stock based and stock issued to officers and directors compensation expense	7,974,000	5,734,000	4,963,000
Net change in accrued interest receivable and other assets	(13,984,000)	25,747,000	13,478,000
Gain from acquisition	(340,000)	(5,628,000)
Net change in other liabilities	13,848,000	(2,641,000)	(2,784,000)
Net cash provided by operating activities	336,538,000	248,874,000	234,402,000
Cash Flows from Investing Activities
Decrease in interest bearing deposits	5,000,000
Purchase of investment securities available-for-sale	(497,787,000)	(339,814,000)	(941,327,000)
Proceeds from maturity and call of investment securities available-for-sale	341,900,000	490,950,000	460,000,000
Proceeds from sale of investment securities available-for-sale	99,899,000	111,704,000	294,000
Purchase of mortgage-backed securities available-for-sale		(267,760,000)
Proceeds from repayment and sale of mortgage-backed securities available-for-sale	105,873,000	71,645,000	758,271,000
Purchase of Federal Home Loan Bank stock	(5,430,000)	(8,160,000)	(1,650,000)
Redemption of Federal Home Loan Bank stock	11,265,000	13,482,000	1,650,000
Redemption of Federal Reserve Bank stock		8,733,000
Net increase in loans	(1,125,623,000)	(963,858,000)	(1,051,952,000)
Purchase of premises and equipment	(6,670,000)	(3,188,000)	(3,523,000)
Proceeds from sales of premises and equipment		5,598,000	12,000
Proceeds from sales of other real estate owned	3,820,000	18,357,000	7,699,000
Increase in investment in affordable housing and alternative energy partnerships	(66,574,000)	(40,284,000)	(82,966,000)
Acquisitions, net of cash acquired		(1,392,000)
Net cash used in investing activities	(1,134,327,000)	(903,987,000)	(853,492,000)
Cash Flows from Financing Activities
Net increase in deposits	1,012,284,000	201,224,000	1,166,044,000
Net decrease in federal funds purchased and securities sold under agreements to repurchase	(100,000,000)	(250,000,000)	(50,000,000)
Advances from Federal Home Loan Bank	5,130,000,000	4,823,000,000	3,555,000,000
Repayment of Federal Home Loan Bank borrowings	(5,030,000,000)	(4,773,000,000)	(3,480,000,000)
Cash dividends	(83,428,000)	(69,888,000)	(59,274,000)
Purchase of treasury stock	(42,648,000)		(54,441,000)
Proceeds from issuance of long-term debt and other borrowings		75,000,000
Repayment of long-term debt and other borrowings	(23,040,000)
Proceeds from shares issued to Dividend Reinvestment Plan	2,822,000	2,528,000	2,277,000
Proceeds from exercise of stock options	838,000	1,094,000	7,661,000
Taxes paid related to net share settlement of RSUs	(3,550,000)	(5,128,000)	(103,000)
Net cash provided by financing activities	863,278,000	4,830,000	1,087,164,000
Increase/(Decrease) in cash and cash equivalents	65,489,000	(650,283,000)	468,074,000
Cash and cash equivalents, beginning of the year	534,801,000	1,185,084,000	717,010,000
Cash and cash equivalents, end of the year	600,290,000	534,801,000	1,185,084,000
Supplemental disclosure of cash flow information
Interest	116,524,000	81,069,000	81,793,000
Income taxes	65,866,000	76,709,000	38,671,000
Non-cash investing and financing activities:
Net change in unrealized holding (loss)/gain on securities available-for-sale, net of tax	(7,949,000)	485,000	3,886,000
Net change in unrealized gain on interest rate swaps designated as cash flow hedges	1,525,000	719,000	825,000
Transfers to other real estate owned from loans held for investment	5,476,000	1,243,000	2,698,000
Loans transferred to loans held for sale		8,000,000	7,953,000
Loans to facilitate the sale of other real estate owned		10,500,000	2,616,000
Issuance of stock related to acquisition		34,862,000
Business Acquired During the Period [Member]
Supplemental disclosure for acquisitions
Cash and cash equivalents		283,932,000
Short-term investments		5,000,000
Securities available-for-sale		88,044,000
FHLB and FRB stock		19,890,000
Loans		705,792,000
Premises and equipment		6,239,000
Cash surrender value of life insurance		46,083,000
Deferred tax assets, net		40,690,000
Core deposit intangible		6,122,000
Accrued interest receivable and other assets		10,689,000
Total assets acquired		1,212,481,000
Deposits		813,888,000
Advances from Federal Home Loan Bank		30,000,000
Accrued interest payable and other liabilities		8,512,000
Total liabilities assumed		852,400,000
Net assets acquired		360,081,000
Cash paid		285,324,000
Fair value of common stock issued		34,862,000
Total consideration paid		$ 320,186,000